The present invention is directed to an improved process for the manufacture of acrylonitrile or methacrylonitrile. In particular, the present invention is directed to an improved process related to the recovery and purification of acrylonitrile or methacrylonitrile produced by the direct reaction of propylene, propane or isobutylene, ammonia and oxygen in the presence of a catalyst.
Typically, recovery and purification of acrylonitrile/methacrylonitrile produced by the direct reaction of a hydrocarbon selected from the group consisting of propane, propylene or isobutylene, ammonia and oxygen in the presence of a catalyst has been accomplished by transporting the reactor effluent containing acrylonitrile/methacrylonitrile to a first column (quench) where the reactor effluent is cooled with a first aqueous stream, transporting the cooled effluent containing acrylonitrile/methacrylonitrile into a second column (absorber) where the cooled effluent is contacted with a second aqueous stream to absorb the acrylonitrile/methacrylonitrile into the second aqueous stream, transporting the second aqueous stream containing the acrylonitrile/methacrylonitrile from the second column to a first distillation column (recovery column) for separation of the crude acrylonitrile/methacrylonitrile from the second aqueous stream, and transporting the separated crude acrylonitrile/methacrylonitrile to a second distillation column (heads column) to remove at least some impurities from the crude acrylonitrile/ methacrylonitrile, and transporting the partially purified acrylonitrile/methacrylonitrile to a third distillation column (product column) to obtain product acrylonitrile/methacrylonitrile. U.S. Pat. Nos. 4,234,510; 3,885,928; 3,352,764; 3,198,750 and 3,044,966 are illustrative of typical recovery and purification processes for acrylonitrile and methacrylonitrile.
Some modifications to the typical recovery and purification process described above have been explored including recycle of the product column bottom stream into the lower portion of the product column via a reboiler. The reboiler is used to reheat the product column bottom stream prior to reentry into the product column. Water addition to the product column including reboiler has been suggested with an attendant observations that corrosion has been a problem in the reboiler tubes.
While the manufacture of acrylonitrile/methacrylonitrile including the recovery and purification have been commercially practiced for years their are still areas in which improvement would have a substantial benefit. One of those areas for improvement is in the substantial elimination or reduction of undesirable polymeric reactions which results in fouling of certain columns over time resulting in the necessity of shut down of the plant for cleaning. The present invention is directed to an improvement in the current acrylonitrile manufacturing process which results in a substantial elimination of fouling in the product column operation thereby substantially increasing the time between shutdowns and cleaning of the plant resulting in a substantial economic benefit during the manufacture of acrylonitrile/methacrylonitrile.